1. Field of the Invention
This invention relates to the field of flow and noise control in an incompressible and/or compressible fluid. More specifically, the invention comprises the use of microjets to reduce noise caused by air flowing over an open cavity such as a sunroof opening or an open window in an automobile.
2. Description of the Related Art
Modern automobiles are designed to provide smooth airflow over their exterior surfaces. Careful design has resulted in a substantial reduction in interior noise—even at highway speeds. Passengers are now accustomed to a relatively quiet environment in which music may be clearly heard and voice communications over cellular telephones are routine. This low ambient noise level is lost, however, when a window or sunroof is opened.
The flow phenomena occurring across an open sunroof and an open window are grossly similar. A sunroof therefore makes a good general example. FIG. 1 shows a sectional elevation view through the roof of a prior art automobile. Roof 20, along with the rest of the automobile's passenger compartment, creates interior 14. Exterior 16 experiences airflow 22 when the car is in motion. Sunroof assembly 10 includes sliding panel 12 and pocket 18 into which the sliding panel retracts when the sunroof is opened.
FIG. 2 shows the same assembly with sliding panel 12 retracted into pocket 18. This creates opening 24, through which air can flow between exterior 16 and interior 14. The result is a dramatic increase in aerodynamic noise inside the cabin. This noise is frequently dominated by “tones” which are pressure pulses centered on discrete frequencies. These cyclic pulses are annoying at best and intolerable at worst. The result is that most vehicle occupants now ride with the windows rolled up and the sunroof closed.
The discrete frequencies produced are likely created by a flow-induced resonance phenomenon. Air flowing over the automobile's exterior tends to lift free from the surface at leading edge 26 of opening 24. The air coming from the leading edge is commonly referred to as a shear layer which separates from the leading edge of the opening and begins to roll up into large-scale rotating structures due to the well-known Kelvin-Helmholtz instability mechanism. When these structures strike the trailing edge of the opening, strong acoustic waves are generated. Under the appropriate conditions, the flow becomes self-excited and significant amplification results.